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Lehninger 7th edition- 13.4

Nelson, D. L., & Cox, M. M. (2017). Principles of Biochemistry 7e. W. H. Freeman. 13.3: Phosphoryl Group Transfers and ATP: Inorganic Polyphosphate Is a Potential Phosphoryl Group Donor.

(This part has been removed from the eighth edition of the book.)

PPK-1 and PPK-2 are essentially catalyzing the same reaction, however, PPK-1 is used when polyphosphate synthesis is thermodynamically favorable and PPK-2 is used when the reverse reaction(ATP synthesis) is favorable. What is the need for having 2 different enzymes if 1 enzyme is enough to catalyze the forward and reverse reaction?

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    $\begingroup$ Welcome to SE Biology. Please indicate the source of the graphic, with date. $\endgroup$
    – David
    Mar 3 at 14:32
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    $\begingroup$ Oops, I forgot that! thanks for reminding me. Nelson, D. L., & Cox, M. M. (2017). Principles of Biochemistry 7e. W. H. Freeman. 13.3: Phosphoryl Group Transfers and ATP: Inorganic Polyphosphate Is a Potential Phosphoryl Group Donor. (This part has been removed from the eighth edition of the book.) $\endgroup$
    – Sadegh Rizi
    Mar 3 at 15:03
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    $\begingroup$ I don't see the mystery. Having 2 enzymes doing opposite things allows for better activity or specificity (each can be much better suited than a "general" catalyst) and regulation, over multiple levels (e.g. localization, transcription, post-translational modification, inhibition, degradation, etc.). Don't you think a single enzyme catalyzing forward and reverse reactions would be rather difficult to regulate, or be very functionally limited? For purposes of ATP production, I think an enzyme strictly producing ATP would have viability over another enzyme simultaneously producing/degrading ATP. $\endgroup$
    – S Pr
    Mar 3 at 15:22
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    $\begingroup$ Well, what you said about the different regulations of the two enzymes makes sense. I'm looking for a more specific answer, however. Also, I have a comment on your last sentence. In reversible reactions, all the enzymes catalyze the forward and reverse reactions simultaneously and equally. And in this case, since these two reactions are reverse of each other, any enzyme catalyzing either of these two reactions would catalyze the other too. The existence of an enzyme that strictly produces ATP contradicts what I have in mind about enzyme function. Am I right? $\endgroup$
    – Sadegh Rizi
    Mar 3 at 16:59
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    $\begingroup$ This is a complex area that I was completely unaware of and know nothing about. The first question that I ask is how you reverse a reaction with a ΔG˚ of -20kJ per/mol? Two possibilities occur to me — either a high ratio of polyphosphate to ATP or that there has been some mistake and the two reactions are not the reverse of each other. The few papers I looked at on PPK2 did not address the thermodynamics. Without further information I cannot be of any help. $\endgroup$
    – David
    Mar 3 at 18:22

1 Answer 1

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The textbook is either incorrect or very poorly worded. You are correct that all enzymes catalyze both both the forward and reverse reactions. The enzyme could only lower the activation energy of this given reaction, affecting both the forward and reverse reaction rates. To drive them in the forward direction, enzymes must couple endergonic reactions (i.e. $\Delta G > 0$) with exergonic reactions (i.e. $\Delta G < 0$).

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  • $\begingroup$ Please add sources to support your answer $\endgroup$
    – AliceD ♦
    Jun 10 at 15:10

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